ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 889 



Localities. — (l) Penkill ; (2) Bargany Pond Burn, Cuddystone Glen. 



Remarks. — Davidson labelled this species L. transver satis, but the muscle-scars 

 and the outline and contour of the shell are quite distinct. The outline more 

 resembles the variety Duvali, but the shell is less transverse and more triangular, 

 and the median part of the pedicle-valve is more swollen and more differentiated 

 from the lateral portions ; the beak is more incurved and more swollen and projects 

 further beyond the hinge-line, and the primary radii are less numerous. From 

 PI. segmentum it also differs in shape and internal characters. The muscle-scars, 

 vascular markings, and median septum are much like those in the shell * from the 

 " Caradoc" of Haverfordwest, referred by Davidson to PI. sericea. 



Plectambonites transversalis (Dalman), var nov. tricostata. 



(Plate XVI, figs. 8-13.) 



1883. Leptaena segmentum, Angelin, Davidson (pars), Mon. Brit. Foss. Brack., vol. v, Silur. Suppl., 

 p. 227. 



Shell transversely elongated, subtriangular to subfusiform, concavo-convex ; 

 cardinal angles acutely pointed at 15°-25°, produced, slightly alate. Pedicle-valve 

 gently convex, most so in middle, rounded, not carinate ; lateral portions flattened, 

 rarely subcylindrical ; beak small, incurved, pointed, not swollen, scarcely projecting ; 

 median portion of valve bearing group of three equidistant strong radii diverging at 

 about 20°-30°, with a very short stout radius (?) on each side of central one arising 

 from beak ; interspaces somewhat depressed, with very fine radial strise ; lateral 

 portions of valve with primary radii wanting or obsolescent, but with faint concentric 

 rugse near cardinal angles. 



Dimensions. — Length, 5"1 mm. ; width, 14'0 mm. 



Horizon. — Saugh Hill Group. 



Locality. — Woodland Point. 



Remarks. — This variety seems to pass into that termed Duvali, but the cardinal 

 angles are not generally cylindrical, and the median group of radii and concentric 

 rugae are usually sufficient to separate it. The two short so-called radii seen near 

 the beak on exteriors of the pedicle-valve seem to correspond to internal ridges, but 

 no interiors of the shell are known. 



Plectambonites ? youngiana (Davidson). 

 (Plate XVI, figs. 14-18.) 



1870. Leptaena transversalis, Dalman, var. youngiana, Davidson, Mon. Brit. Foss. Brack., vol. iii, 



pt. vii, p. 320, pi. xlvii, figs. 19, 20. 

 1883. Leptaena youngiana, Davidson, ibid., vol. v, Silur. Suppl., p. 168, pi. xii, figs. 14-16. 



Davidson did not describe the internal characters of this species, though he 

 figured in his Supplement {op. cit., pi. xii, fig. 1G) one interior of a brachial valve 

 * Davidson, op. cit., vol. iii, pi. xlviii, figs. 19, 19a (non cet.). 



